服务器执行命令过程：

一、服务器结构：

redisServer以及在第一篇文章[1]中讲过，这里只介绍后两个结构：

/* With multiplexing we need to take per-client state.
 * Clients are taken in a liked list. */
typedef struct redisClient {
    int fd;
    redisDb *db;
    int dictid;
    robj *name;             /* As set by CLIENT SETNAME */
    sds querybuf;
    int argc;
    robj **argv;
    struct redisCommand *cmd, *lastcmd;
    int reqtype;
    int multibulklen;       /* number of multi bulk arguments left to read */
    long bulklen;           /* length of bulk argument in multi bulk request */
    list *reply;
    unsigned long reply_bytes; /* Tot bytes of objects in reply list */
    int sentlen;            /* Amount of bytes already sent in the current
   
    /* Response buffer */
    int bufpos;
    char buf[REDIS_REPLY_CHUNK_BYTES];
} redisClient;

• L8：输入缓冲区，存放读取的命令请求；
• L21：输出缓冲区，结果响应；

struct redisCommand {
    char *name;
    redisCommandProc *proc;
    int arity;
    char *sflags; /* Flags as string representation, one char per flag. */
    int flags;    /* The actual flags, obtained from the 'sflags' field. */
    /* Use a function to determine keys arguments in a command line. */
    redisGetKeysProc *getkeys_proc;
    /* What keys should be loaded in background when calling this command? */
    int firstkey; /* The first argument that's a key (0 = no keys) */
    int lastkey;  /* The last argument that's a key */
    int keystep;  /* The step between first and last key */
    long long microseconds, calls;
};

• L3：命令处理例程；

• redisCommand就被存放在一个列表中：

  struct redisCommand redisCommandTable[] = {
      {"get",getCommand,2,"r",0,NULL,1,1,1,0,0},
      {"set",setCommand,-3,"wm",0,noPreloadGetKeys,1,1,1,0,0},
      {"setnx",setnxCommand,3,"wm",0,noPreloadGetKeys,1,1,1,0,0},
      // ...
  }
  

二、读取客户端命令请求：

​ Redis的网络事件处理是基于Reactor模型的，因此客户端的命令请求就是对应连接的fd上的监听事件触发：

redisClient *createClient(int fd) {
    redisClient *c = zmalloc(sizeof(redisClient));

    /* passing -1 as fd it is possible to create a non connected client.
     * This is useful since all the Redis commands needs to be executed
     * in the context of a client. When commands are executed in other
     * contexts (for instance a Lua script) we need a non connected client. */
    if (fd != -1) {
        anetNonBlock(NULL,fd);
        anetEnableTcpNoDelay(NULL,fd);
        if (server.tcpkeepalive)
            anetKeepAlive(NULL,fd,server.tcpkeepalive);
        /* 监听客户端的可读事件，设置回调 */
        if (aeCreateFileEvent(server.el,fd,AE_READABLE,
            readQueryFromClient, c) == AE_ERR)
        {
            close(fd);
            zfree(c);
            return NULL;
        }
    }
	// ...
}

• L14：监听客户端的可读事件，并设置回调函数readQueryFromClient；

void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) {
    redisClient *c = (redisClient*) privdata;
    int nread, readlen;
    size_t qblen;
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(mask);

    server.current_client = c;
    readlen = REDIS_IOBUF_LEN;
    /* If this is a multi bulk request, and we are processing a bulk reply
     * that is large enough, try to maximize the probability that the query
     * buffer contains exactly the SDS string representing the object, even
     * at the risk of requiring more read(2) calls. This way the function
     * processMultiBulkBuffer() can avoid copying buffers to create the
     * Redis Object representing the argument. */
    if (c->reqtype == REDIS_REQ_MULTIBULK && c->multibulklen && c->bulklen != -1
        && c->bulklen >= REDIS_MBULK_BIG_ARG)
    {
        int remaining = (unsigned)(c->bulklen+2)-sdslen(c->querybuf);

        if (remaining < readlen) readlen = remaining;
    }

    qblen = sdslen(c->querybuf);
    if (c->querybuf_peak < qblen) c->querybuf_peak = qblen;
    c->querybuf = sdsMakeRoomFor(c->querybuf, readlen);
    /* 将命令读入客户端querybuf（输入缓冲区）中 */
    nread = read(fd, c->querybuf+qblen, readlen);
    if (nread == -1) {
        if (errno == EAGAIN) {
            nread = 0;
        } else {
            redisLog(REDIS_VERBOSE, "Reading from client: %s",strerror(errno));
            freeClient(c);
            return;
        }
    } else if (nread == 0) {
        redisLog(REDIS_VERBOSE, "Client closed connection");
        freeClient(c);
        return;
    }
    if (nread) {
        /* 更新sds的len和free属性 */
        sdsIncrLen(c->querybuf,nread);
        c->lastinteraction = server.unixtime;
        /* 如果客户端是master，即本端是slave；
         * 本端收到的命令请求是master发过来用于部分重同步的，
         * 所以需要更新reploff（replication offset） */
        if (c->flags & REDIS_MASTER) c->reploff += nread;
    } else {
        server.current_client = NULL;
        return;
    }
    /* 写入数据过大时关闭客户端 */
    if (sdslen(c->querybuf) > server.client_max_querybuf_len) {
        sds ci = getClientInfoString(c), bytes = sdsempty();

        bytes = sdscatrepr(bytes,c->querybuf,64);
        redisLog(REDIS_WARNING,"Closing client that reached max query buffer length: %s (qbuf initial bytes: %s)", ci, bytes);
        sdsfree(ci);
        sdsfree(bytes);
        freeClient(c);
        return;
    }
    /* 从输入缓存中读取内容，执行命令 */
    processInputBuffer(c);
    server.current_client = NULL;
}

• L49：注意主从复制[1]时，master是slave的服务器，也是slave的客户端；

• L66：第三、四节分析命令分析和执行过程；

三、命令分析：

void processInputBuffer(redisClient *c) {
    /* Keep processing while there is something in the input buffer */
    while(sdslen(c->querybuf)) {
        /* Immediately abort if the client is in the middle of something. */
        if (c->flags & REDIS_BLOCKED) return;

        /* REDIS_CLOSE_AFTER_REPLY closes the connection once the reply is
         * written to the client. Make sure to not let the reply grow after
         * this flag has been set (i.e. don't process more commands). */
        if (c->flags & REDIS_CLOSE_AFTER_REPLY) return;

        /* Determine request type when unknown. */
        if (!c->reqtype) {
            if (c->querybuf[0] == '*') {
                c->reqtype = REDIS_REQ_MULTIBULK;
            } else {
                c->reqtype = REDIS_REQ_INLINE;
            }
        }

        if (c->reqtype == REDIS_REQ_INLINE) {
            if (processInlineBuffer(c) != REDIS_OK) break;
        } else if (c->reqtype == REDIS_REQ_MULTIBULK) {
            if (processMultibulkBuffer(c) != REDIS_OK) break;
        } else {
            redisPanic("Unknown request type");
        }

        /* Multibulk processing could see a <= 0 length. */
        if (c->argc == 0) {
            resetClient(c);
        } else {
            /* Only reset the client when the command was executed. */
            if (processCommand(c) == REDIS_OK)
                resetClient(c);
        }
    }
}

• L13：如今Redis基本上都用 L15的协议的格式[2]；
• L21~L27：根据协议格式，分析缓冲区中命令；
• L34：执行命令，第四节介绍；

int processMultibulkBuffer(redisClient *c) {
    char *newline = NULL;
    int pos = 0, ok;
    long long ll;

    /* 假设命令为:
     *     *3\r\n$3\r\nSET\r\n...
     *     |
     *     |
     * querybuf
     * */
    if (c->multibulklen == 0) {
        /* The client should have been reset */
        redisAssertWithInfo(c,NULL,c->argc == 0);

        /* Multi bulk length cannot be read without a \r\n */
        newline = strchr(c->querybuf,'\r');
        if (newline == NULL) {
            if (sdslen(c->querybuf) > REDIS_INLINE_MAX_SIZE) {
                addReplyError(c,"Protocol error: too big mbulk count string");
                setProtocolError(c,0);
            }
            return REDIS_ERR;
        }

        /* Buffer should also contain \n */
        if (newline-(c->querybuf) > ((signed)sdslen(c->querybuf)-2))
            return REDIS_ERR;

        /* We know for sure there is a whole line since newline != NULL,
         * so go ahead and find out the multi bulk length. */
        redisAssertWithInfo(c,NULL,c->querybuf[0] == '*');        
         /* 1.获取参数个数
          * 假设命令为:
          *     *3\r\n$3\r\nSET\r\n...
          *     |  | 
          *     |  |
          *    qb  newline
          * 
          *  ll即为3 */
        ok = string2ll(c->querybuf+1,newline-(c->querybuf+1),&ll);
        if (!ok || ll > 1024*1024) {
            addReplyError(c,"Protocol error: invalid multibulk length");
            setProtocolError(c,pos);
            return REDIS_ERR;
        }

        pos = (newline-c->querybuf)+2;
        if (ll <= 0) {
            sdsrange(c->querybuf,pos,-1);
            return REDIS_OK;
        }
        /* 参数个数 */
        c->multibulklen = ll;

        /* Setup argv array on client structure */
        if (c->argv) zfree(c->argv);
        c->argv = zmalloc(sizeof(robj*)*c->multibulklen);
    }

    redisAssertWithInfo(c,NULL,c->multibulklen > 0);
    while(c->multibulklen) {
        /* Read bulk length if unknown */
        if (c->bulklen == -1) {
            newline = strchr(c->querybuf+pos,'\r');
            if (newline == NULL) {
                if (sdslen(c->querybuf) > REDIS_INLINE_MAX_SIZE) {
                    addReplyError(c,"Protocol error: too big bulk count string");
                    setProtocolError(c,0);
                }
                break;
            }

            /* Buffer should also contain \n */
            if (newline-(c->querybuf) > ((signed)sdslen(c->querybuf)-2))
                break;

            if (c->querybuf[pos] != '$') {
                addReplyErrorFormat(c,
                    "Protocol error: expected '$', got '%c'",
                    c->querybuf[pos]);
                setProtocolError(c,pos);
                return REDIS_ERR;
            }
            /* 2.获取参数长度
             * 假设命令为:
             *     *3\r\n$3\r\nSET\r\n...
             *     |     |  |
             *     |     |  |
             *    qb    pos newline
             * 
             *  ll即为$之后的3 */
            ok = string2ll(c->querybuf+pos+1,newline-(c->querybuf+pos+1),&ll);
            if (!ok || ll < 0 || ll > 512*1024*1024) {
                addReplyError(c,"Protocol error: invalid bulk length");
                setProtocolError(c,pos);
                return REDIS_ERR;
            }

            /* 移动到参数开头 
             * 3.获取参数
             * 假设命令为:
             *     *3\r\n$3\r\nSET\r\n...
             *     |         |  
             *     |         |  
             *    qb        pos 
             * */
            pos += newline-(c->querybuf+pos)+2;
            if (ll >= REDIS_MBULK_BIG_ARG) {
                size_t qblen;

                /* If we are going to read a large object from network
                 * try to make it likely that it will start at c->querybuf
                 * boundary so that we can optimize object creation
                 * avoiding a large copy of data. */
                sdsrange(c->querybuf,pos,-1);
                pos = 0;
                qblen = sdslen(c->querybuf);
                /* Hint the sds library about the amount of bytes this string is
                 * going to contain. */
                if (qblen < ll+2)
                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-qblen);
            }
            /* 参数长度 */
            c->bulklen = ll;
        }

        /* Read bulk argument */
        if (sdslen(c->querybuf)-pos < (unsigned)(c->bulklen+2)) {
            /* Not enough data (+2 == trailing \r\n) */
            break;
        } else {
            /* Optimization: if the buffer contains JUST our bulk element
             * instead of creating a new object by *copying* the sds we
             * just use the current sds string. */
            if (pos == 0 &&
                c->bulklen >= REDIS_MBULK_BIG_ARG &&
                (signed) sdslen(c->querybuf) == c->bulklen+2)
            {
                c->argv[c->argc++] = createObject(REDIS_STRING,c->querybuf);
                sdsIncrLen(c->querybuf,-2); /* remove CRLF */
                c->querybuf = sdsempty();
                /* Assume that if we saw a fat argument we'll see another one
                 * likely... */
                c->querybuf = sdsMakeRoomFor(c->querybuf,c->bulklen+2);
                pos = 0;
            } else {
                /* 不进行优化，得到参数，pos向后移动 */
                c->argv[c->argc++] =
                    createStringObject(c->querybuf+pos,c->bulklen);
                pos += c->bulklen+2;
            }
            c->bulklen = -1;
            c->multibulklen--;
        }
    }

    /* Trim to pos */
    /* 将pos前（缓冲区已被读取的部分）删除 */
    if (pos) sdsrange(c->querybuf,pos,-1);

    /* We're done when c->multibulk == 0 */
    if (c->multibulklen == 0) return REDIS_OK;

    /* Still not read to process the command */
    return REDIS_ERR;
}

• L136~L136：缓冲区中只有一个参数时进行优化，避免对querybuf的拷贝；

四、命令执行：

processCommand() 在进行一些校验以后，会执行call()函数。官方注释将call函数称作 the **core** of Redis execution of a command。

/* Call() is the core of Redis execution of a command */
void call(redisClient *c, int flags) {
    long long dirty, start = ustime(), duration;
    int client_old_flags = c->flags;

    /* Sent the command to clients in MONITOR mode, only if the commands are
     * not generated from reading an AOF. */
    if (listLength(server.monitors) &&
        !server.loading &&
        !(c->cmd->flags & REDIS_CMD_SKIP_MONITOR))
    {
        replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc);
    }

    /* Call the command. */
    /*to_do why??； */
    c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL);
    redisOpArrayInit(&server.also_propagate);
    dirty = server.dirty;
    c->cmd->proc(c);
    /* 当前指令进行了多少次修改； */
    dirty = server.dirty-dirty;
    duration = ustime()-start;

    /* When EVAL is called loading the AOF we don't want commands called
     * from Lua to go into the slowlog or to populate statistics. */
    if (server.loading && c->flags & REDIS_LUA_CLIENT)
        flags &= ~(REDIS_CALL_SLOWLOG | REDIS_CALL_STATS);

    /* If the caller is Lua, we want to force the EVAL caller to propagate
     * the script if the command flag or client flag are forcing the
     * propagation. */
    if (c->flags & REDIS_LUA_CLIENT && server.lua_caller) {
        if (c->flags & REDIS_FORCE_REPL)
            server.lua_caller->flags |= REDIS_FORCE_REPL;
        if (c->flags & REDIS_FORCE_AOF)
            server.lua_caller->flags |= REDIS_FORCE_AOF;
    }

    /* Log the command into the Slow log if needed, and populate the
     * per-command statistics that we show in INFO commandstats. */
    /* 记录慢查询日志； */
    if (flags & REDIS_CALL_SLOWLOG && c->cmd->proc != execCommand)
        slowlogPushEntryIfNeeded(c->argv,c->argc,duration);
    if (flags & REDIS_CALL_STATS) {
        c->cmd->microseconds += duration;
        c->cmd->calls++;
    }

    /* Propagate the command into the AOF and replication link */
    /* 向从服务器和AOF文件进行命令传播； */
    if (flags & REDIS_CALL_PROPAGATE) {
        int flags = REDIS_PROPAGATE_NONE;

        if (c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL;
        if (c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF;
        if (dirty)
            flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF);
        if (flags != REDIS_PROPAGATE_NONE)
            propagate(c->cmd,c->db->id,c->argv,c->argc,flags);
    }

    /* Restore the old FORCE_AOF/REPL flags, since call can be executed
     * recursively. */
    /* to_do why??； */
    c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL);
    c->flags |= client_old_flags & (REDIS_FORCE_AOF|REDIS_FORCE_REPL);

    /* Handle the alsoPropagate() API to handle commands that want to propagate
     * multiple separated commands. */
    if (server.also_propagate.numops) {
        int j;
        redisOp *rop;

        for (j = 0; j < server.also_propagate.numops; j++) {
            rop = &server.also_propagate.ops[j];
            propagate(rop->cmd, rop->dbid, rop->argv, rop->argc, rop->target);
        }
        redisOpArrayFree(&server.also_propagate);
    }
    server.stat_numcommands++;
}

• L20：客户端命令请求的执行；
• L21：dirty计数器更新，记录了上一次成功执行SAVE or BGSAVE命令后，服务器对数据库状态的修改次数；
• L52~L61：AOF和主从复制相关的命令传播，todo；

​ 下面展示一个c->cmd->proc()执行的实例setCommand() -> setGenericCommand：

void setGenericCommand(redisClient *c, int flags, robj *key, robj *val, robj *expire, int unit, robj *ok_reply, robj *abort_reply) {
    long long milliseconds = 0; /* initialized to avoid any harmness warning */

    if (expire) {
        if (getLongLongFromObjectOrReply(c, expire, &milliseconds, NULL) != REDIS_OK)
            return;
        if (milliseconds <= 0) {
            addReplyError(c,"invalid expire time in SETEX");
            return;
        }
        if (unit == UNIT_SECONDS) milliseconds *= 1000;
    }

    if ((flags & REDIS_SET_NX && lookupKeyWrite(c->db,key) != NULL) ||
        (flags & REDIS_SET_XX && lookupKeyWrite(c->db,key) == NULL))
    {
        addReply(c, abort_reply ? abort_reply : shared.nullbulk);
        return;
    }
    /* 对客户端当前使用的server的数据库（操作c->db->dict）的操作在这里； */
    setKey(c->db,key,val);
    /* dirty计数器； */
    server.dirty++;
    /* 设置过期时间（操作c->db->expires） */
    if (expire) setExpire(c->db,key,mstime()+milliseconds);
    /* 订阅通知机制，to_do； */
    notifyKeyspaceEvent(REDIS_NOTIFY_STRING,"set",key,c->db->id);
    if (expire) notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,
        "expire",key,c->db->id);
    addReply(c, ok_reply ? ok_reply : shared.ok);
}

• L26：订阅通知机制[3] [4]，todo；

四、结果响应:

​ 主要通过addReply*系列函数，修改c->buf输出缓冲区，并且通过prepareClientToWrite在事件循环中监听了可写事件（回调函数完成取消监听，因为Redis使用EPOLL LT模式）。以后可能会详细分析；

参考：

1. Redis源码分析--主从复制 - macguz - 博客园 (cnblogs.com)

2. REDIS protocol -- Redis中国用户组（CRUG）

3. Redis源码解析(7) 发布订阅机制_李兆龙的博客-CSDN博客

4. 10Redis键空间通知(keyspace notifications)_程序员的自我修养-CSDN博客_redis键空间通知